Cytokinesis in Trypanosoma brucei, a parasitic protozoan and the causative agent of human sleeping sickness, is known to be totally different from that in its human host. The cleavage plane in a dividing trypanosome is not defined by the central spindle like in yeasts and animals, but rather by the position of the new flagellum and the flagellum attachment zone (FAZ). Consequently, cytokinesis in trypanosomes is initiated from the anterior tip of the new FAZ, and the cleavage furrow ingresses unidirectionally along the long axis from the anterior towards the posterior end of the cell. Strikingly, an actomyosin contractile ring is not formed in trypanosomes, indicating that trypanosome cleavage furrow may possess an unusual structure with novel components. Despite these differences, however, both Aurora-like and Polo-like kinases are implicated in cytokinesis initiation in trypanosomes (Hammarton et al., 2007; Li and Wang, 2006; Tu et al., 2006). The Aurora kinase homolog in trypanosomes, TbAUK1, forms a unique chromosomal passenger complex (CPC) that plays essential roles in chromosome segregation and cytokinesis initiation (Li et al., 2008a). The CPC exhibits a dynamic localization during mitosis and cytokinesis by migrating from chromatins to the central spindle during metaphase-anaphase transition and from the central spindle to the anterior tip of the new FAZ during mitosis-cytokinesis transition (Li et al., 2008a; Li et al., 200). The Polo-like kinase homolog in trypanosomes, TbPLK, is also concentrated at the anterior tip of the new FAZ where it promotes cytokinesis initiation (de Graffenried et al., 2008; Kumar and Wang, 2006; Umeyama and Wang, 2008). The downstream factors of the CPC and TbPLK at the anterior tip of the new FAZ are not identified, and trypanosomes appear to lack most of the partner protein homologs of the CPC and Polo-like kinase, suggesting that trypanosomes may have evolved distinct CPC- and PLK-mediated pathways for cytokinesis. The current proposal is built upon the essential roles of TbAUK1 and TbPLK in cytokinesis, and aims to address the following questions. (1). How is cytokinesis initiation regulated by TbAUK1 and TbPLK? We hypothesize that TbAUK1 and TbPLK cooperate to regulate cytokinesis initiation by forming a complex or regulating some common downstream factors at the anterior tip of the new FAZ. Our central hypothesis is that after fulfilling its essential function in mitosis TbAUK1 migrates to th anterior tip of the new FAZ where it cooperates with TbPLK and other novel proteins to initiate cytokinesis. (2). How is TbAUK1 activity and subcellular localization regulated during mitosis-cytokinesis transition and during cytokinesis progression? This is still poorly understood, mainly due to the presence of a novel CPC in trypanosomes and the unique dynamic trans-localization of TbAUK1 during mitosis and cytokinesis. (3). What is the structure-function relationship of the CPC and what are the roles of the two novel CPC components, TbCPC1 and TbCPC2? Our hypothesis is that TbCPC1 and/or TbCPC2 are the mediator(s) of TbAUK1 localization and are also likely the activator(s) of TbAUK1. Through molecular, cell biological, chemical genetic, and biochemical means, our overall goal in this proposal is to understand the mechanistic role of TbAUK1 and TbPLK in cytokinesis initiation, the regulation of TbAUK1 by its novel partners TbCPC1 and TbCPC2. The long-term goal of my laboratory is to delineate the regulatory networks that control mitosis and cytokinesis in T. brucei, which will facilitate our fundamental understanding of the molecular basis of mitosis, mitosis-cytokinesis coordination, and cytokinesis that is different from the commonly recognized cell division through the constriction of an actomyosin contractile ring. The outcome from these studies would not only have important biological significance, but also could provide novel targets for anti-trypanosomiasis chemotherapy.